The invention relates to compressors, and more particularly to valve arrangements for controlling the flow of fluid through compressors.
It is known to use positive displacement compressors, and more specifically screw compressors, to compress fluids. The rotors or screws of a screw compressor are susceptible to backward rotation when the compressor is stopped because the pressure differential between the discharge side of the compressor and the suction side of the compressor naturally tends to equalize over the rotors. While the compressors can be designed to handle such backward rotation of the rotors, the noise generated by the backward-turning rotors is undesirable.
To prevent pressure equalization over the compressor, and the resultant backward rotation of the rotors, it is known to use check valves. For the purposes of this description, the compressor is described as being part of a temperature control system, however, it is to be understood that the compressor need not be used in conjunction with a temperature control system. FIG. 1 schematically illustrates a prior art refrigeration system 10. The system 10 includes a compressor (represented by the dashed box 14) having two screws or rotors 16 and a discharge line 18 through which high-pressure refrigerant and lubricating oil exit the rotors 16 at the discharge end of the compressor 14. The discharge line 18 communicates with an oil separator 22 that separates the oil from the high-pressure refrigerant. The oil returns to an oil sump 26 where it can be reintroduced into the rotors 16 via an oil supply line 30. The high-pressure refrigerant exits the compressor 14 through the oil separator 22 and travels to a condenser 34. After exiting the condenser 34, the condensed refrigerant passes through an expansion valve 38 before reaching an evaporator 42. From the evaporator 42, the low-pressure refrigerant returns to the compressor 14 and the refrigeration cycle repeats.
As seen in FIG. 1, a check valve 46 is located at the suction end of the compressor 14. The check valve 46 prevents high-pressure refrigerant from flowing back through the rotors 16 toward the lower pressure at the suction end of the compressor 14, and thereby prevents backward rotation of the rotors 16. An advantage of locating the check valve 46 at the suction end of the compressor 14 is that when the compressor 14 is shut down there is no pressure equalization over the oil system so oil will not be displaced from the oil sump 26 into the rotors 16. Rather, the pressure is equalized downstream of the discharge end of the compressor 14.
The disadvantage of locating the check valve 46 as shown in FIG. 1 is that the check valve 46 must be relatively large to prevent the high-pressure gas from taking its natural equalization path over the compressor to the lower-pressure suction end. Additionally, any pressure drop caused by the check valve 46 while the system is operating will substantially reduce the system""s capacity.
FIG. 2 shows another prior art refrigeration system 10xe2x80x2, with like parts having like reference numerals. In the system 10xe2x80x2, a check valve 50 is located downstream of the oil separator 22. The check valve 50 prevents high-pressure refrigerant from flowing back into the oil separator 22 and the rotors 16. Locating the check valve 50 downstream of the oil separator 22 also provides advantages. First, the check valve 50 can be relatively small because the high-pressure refrigerant will naturally flow toward the lower-pressure environment of the condenser 34. In other words, because the high-pressure refrigerant downstream of the oil separator 22 does not tend to flow back into the oil separator 22, the check valve 50 can be relatively small. Additionally, any pressure drop caused by the check valve 50 while the system is operating will only affect power consumption and not system capacity.
The disadvantage with the location shown in FIG. 2 is that, in most situations, the volume of high-pressure refrigerant in the oil separator 22 is still large enough to cause noticeable backward rotation of the compressor rotors 16 as the pressure equalizes over the compressor 14. To alleviate this problem, it is known to add a second check valve 54 at the suction end of the compressor 14. This second check valve 54 operates in the manner described above with respect to the check valve 46, so that the volume of high-pressure refrigerant in the oil separator 22 does not flow back through the rotors 16. While this configuration creates maximum isolation of the compressor 14 from the remaining components of the refrigeration system 10xe2x80x2, it necessitates the use of two check valves 50 and 54, and adds to the cost of the refrigeration system 10xe2x80x2.
FIG. 3 shows yet another prior art refrigeration system 10xe2x80x3, with like parts having like reference numerals. A check valve 58 is located at the discharge end of the compressor 14, between the rotors 16 and the oil separator 22. When the compressor 14 stops running, the pressure between the discharge end and the suction end of the compressor 14 equalizes over the oil system via the oil supply line 30. The disadvantage with this check valve location is that when the pressure is equalized over the oil system, oil from the oil sump 26 is displaced into the rotors 16, the bearings (not shown), the gears (not shown), and the seal cavities (not shown). Too much oil in the rotors 16 makes the compressor 14 difficult to start and reduces the overall life of the compressor 14. For example, since oil is not a compressible medium, too much oil in the rotors 16 could create a hydraulic lock situation. To overcome these problems, it has been known to place a solenoid valve 62 in the oil supply line 30. The solenoid valve 62 is opened when the compressor 14 is running and closed when the compressor 14 is stopped.
One disadvantage with using the solenoid valve 62 is the additional cost. Furthermore, failure of the solenoid valve 62 could cause problems. For example, if the solenoid valve 62 is stuck closed when the compressor 14 is running, the compressor 14 will not get lubrication and will eventually seize. If the solenoid valve 62 is stuck open when the compressor 14 is stopped, oil will be displaced to the rotors 16, creating the difficult starting conditions that the solenoid valve 62 was intended to prevent.
The present invention provides a valve arrangement that offers many of the advantages discussed above, without most of the disadvantages. More particularly, the invention provides a valve arrangement having a single, relatively small valve located in the discharge line of the compressor. When the compressor is running, the valve provides the necessary fluid communication between the compressor and the oil separator. When the compressor is shut down, the valve blocks fluid communication between the rotors and the oil separator to prevent the high-pressure fluid from flowing back over the rotors.
In addition, the valve arrangement also prevents displacement of oil to the rotors when the compressor shuts down, and does so without the use of a solenoid valve in the oil supply line. To accomplish this, the valve arrangement includes a bleed line communicating between the oil supply line and the discharge line. When the compressor is not operating, the valve and the bleed line provide a pathway for the high and low pressure fluid to equalize over the oil cavities in the compressor while short-circuiting the oil separator and the oil sump. Because the pressure equalization does not occur over the oil sump, substantially no oil is displaced to the rotors.
The valve provides selective communication between the discharge end of the compressor, the oil separator, and the bleed line. A movable member in the valve responds to system pressure so that when the compressor is running, the movable member is in a first position that allows communication between the discharge end of the compressor and the oil separator, while blocking communication between the discharge end of the compressor and the bleed line. When the compressor is stopped, the movable member in the valve moves to a second position that blocks communication between the discharge end of the compressor and the oil separator, and allows communication between the discharge end of the compressor and the bleed line.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.